I'm a life long stutterer in my 40s. It took me far too long to realize that trying to hide it is more than useless, it's counterproductive. She'll find out eventually and be stunned, confused and get self-conscious about her reaction and it will all go to hell . Be upfront about it. You'll be surprised how a lot of girls will actually find it endearing. And if she recoils? Well, she's not the kind of girl you'd want to be with anyway.